S-NPP and NOAA-20 VIIRS thermal emissive bands diurnal F-factor oscillations impacts on Earth view retrievals

The VIIRS instruments onboard the Suomi National Polar-orbiting Partnership (S-NPP) and NOAA-20 (N20) spacecraft have been operating and providing public data records to the scientific and research communities for close to one decade and 3 years, respectively. VIIRS has 7 Thermal Emissive Bands (TEB), calibrated on-orbit using observations from its on-board blackbody, with central wavelengths that range from 3.7 μm to 12.0 μm. While the on-orbit performance of the VIIRS TEB for both instruments have been evaluated in the past and shown to be quite stable (maximum gain changes of 3.0 % and 0.5 % for S-NPP and N20 VIIRS, respectively, and noise equivalent difference temperature values well within specification), diurnal F-factor (measure of gain change) oscillations associated with orbital changes are common. This manuscript focuses on evaluating the VIIRS TEB F-factor diurnal oscillations impact on the Earth view (EV) retrievals. Daily VIIRS F-factor data is fitted using a sinusoidal regression to find the F-factor oscillation amplitude for each TEB. Afterwards, this amplitude is added to typical F-factor values and used in an analytical model to compute its impact on the EV brightness temperature (BT) for all the VIIRS TEB. Results show that the diurnal F-factor oscillations cause EV BT changes of up to 0.070 K (bands I5 and M16) and 0.045 K (M16) for S-NPP and N20 VIIRS, correspondingly. The findings and methodology used in this study present a possible mitigation strategy for the correction of EV BT changes caused by diurnal gain oscillations in the Level 1B data.